Methods and systems for data transmission by means of a closed transmission loop are known. Prior art within this area is characterized by the fact that it is directed to data transmission on a so called high level, i.e. large data volumes which are transmitted at high bit rates.
Transmission of data from a superior unit to subordinated units of the loop is generally carried out such that the superior unit calls for the subordinated units in sequence and eventually in accordance with a priority order of the sub-units. In this manner a point-to-point connection is provided between the superior unit and the sub-unit, which will then receive the actual amount of information from the superior unit. When this transmission is ended a new point-to-point connection to the sub-unit in turn to be supplied with data is established. Eventually the priority of the sub-units may be renewed after each transmission which is completed. Establishing successively in this manner the different connections will bring with it periods of waiting at the sub-units and therefore make necessary buffer memories of a sufficient capacity at the superior unit as well as the subordinated units.
Transmission in the other direction, i.e. from the subordinated units to the superior unit, is generally carried out such that the superior unit transmits one so called polling character having the task to initiate a connection between the superior unit and subordinated units for which a contact is desirable.
Different methods to obtain this connection are known. According to one known method the polling character is used for the collection of the addresses of the subordinated units for which a contact is desirable. Then the superior unit initiates the establishment of point-to-point connections to the subordinated units in question, and there the connections are eventually established in accordance with an actual priority order of the sub-units, which are then allowed to transmit the amounts of data in question.
According to one further known method said polling character is used for the establishment of such a point-to-point connection between the superior unit and the sub-units for which a contact is desirable in the order in which said sub-units are positioned along the loop, said polling character being stored temporarily in the sub-unit during the elapse of time of the data transmission in order to be sent forward on the loop at the end of the transmission and then to initiate the next desirable connection in turn.
From this is clear that transmission in this direction will also bring with it inevitable periods of waiting at the sub-units, the time lengths of which are directly dependent on the actual data volumes to be transmitted. Therefore buffer memories are also required for transmission from the sub-units.
Normally in systems of the type disclosed above relatively large amounts of data are transmitted per transmission. Every transmission is terminated by sending an acknowledgement message from a receiving unit, an acknowledgement character, "ACK-character", being sent in case the data volume transmitted has been received correctly and a "NACK-character" being sent in case the data is received incorrectly. When the transmitting unit receives a NACK character a re-transmission of the data volume is initiated which will bring with it time delays, which particularly in case of large data volumes may amount to values which are important and thereby extend the time of waiting at the different units.